As disclosed in Patent Document 1, for example, an EGR valve device has one or more exhaust gas inlet ports and one or more exhaust gas outlet ports, and includes: a valve housing forming an exhaust gas passage connected to those exhaust gas inlet ports and exhaust gas outlet ports; valve seats disposed on the inner periphery of the valve housing; a valve shaft assembled to the valve housing; and valves that are mounted to the valve shaft and simultaneously abut against their respective valve seats when the valve shaft moves in a direction; when the valves seated against their respective valve seats are opened, the exhaust gas exhausted from the exhaust gas outlet port is circulated to be aspirated to an engine. As shown in FIG. 7, for example, a valve seat 140 of the EGR valve device is a flat plate made of generally disk-shaped stainless steel or the like having a hole formed at the center thereof, and is secured to a housing 110 made of aluminum or the like by integrally fixing the peripheral portion of the valve seat to the housing by insert-casing or the equivalent method.
In such an EGR valve device, the temperature of an exhaust gas passage sometimes reaches about −40° C. in a low temperature, and reaches from about 100° C. to 700° C. in a high temperature according to the conditions of the engine; there can develop a large gap between components that are insert-cast by materials having a linear expansion coefficient different from each other. For example, in a case where placing the components under a low temperature=−40° C. for one hour and a high temperature=+350° C. for one hour is defined as one cycle, when 100 cycles of this are carried out, there may develop a significant gap s0 of about 100 μm between the insert-cast components employing the materials having a linear expansion coefficient different from each other, and the gap s0 may cause the chatter of the valve seat 140.